The present invention relates to a surface-mount (or surface mountable) coil; and, more particularly, to an electrode structure thereof and a method of making same.
Nowadays, miniaturized chip type electronic components are extensively employed in high density surface mounting on a printed circuit board by using a chip mounter. Referring to FIGS. 10 and 11, there are illustrated a partial cut-away view and a cross sectional view of a conventional winding type surface mountable chip coil 10 having a wiring wound around a core thereof. The coil 10 typically includes a drum-shaped core 4 having a body portion 1 and raised portions 2, 3 integrally formed at two opposite ends of the body portion 1; a winding wire 5 wound around the body portion 1; base electrodes 6-1, 6-2 disposed on two end surfaces 2b, 3b and also on parts of peripheral surfaces 2a, 3a of the raised portions 2, 3, two ends (not shown) of the winding wire 5 being connected to the base electrodes 6-1, 6-2; an encapsulating member 7 covering the whole structure excepting parts of the base electrodes 6-1, 6-2 at the central regions of the end surfaces 2b, 3b of the raised portions 2, 3; and terminal electrodes 8-1, 8-2 covering exposed base electrodes 6-1a, 6-2a up to portions of the encapsulating member 7 on the peripheral surfaces 2a and 3a. 
In the surface mountable coil 10 illustrated above, the drum-shaped core 4, to which the base electrodes 6-1, 6-2 can be directly attached, is made of a magnetic material, e.g., nickel-zinc based ferrite of a high resistivity, or an insulating material, e.g., alumina. The base electrodes 6-1, 6-2 are conductive layers, each including therein Ag, Agxe2x80x94Pt or Cu film formed by dip-baking or plating, and a conductive material, e.g., Ni/Sn or Sn alloy formed thereon. The winding wire 5 is a conductive wire coated with an insulating film, e.g., polyurethane, polyamideimide, and the like. with a diameter of 0.03xcx9c0.15 mm and the respective end portions thereof are connected to the base electrodes 6-1, 6-2 on the peripheral surfaces 2a, 3a of the raised portions 2, 3 by means of welding, thermocompression bonding, ultrasonic vibration, or a combination thereof. The encapsulating member 7 is formed by injection molding of an epoxy based synthetic resin.
After forming the encapsulating number 7, the terminal electrodes 8-1, 8-2 are formed on the regions corresponding to the end surfaces 2b, 3b and the peripheral surfaces 2a, 3a of the raised portions 2, 3, respectively, and the finished structure is shaped to provide the thin miniaturized surface mountable coil 10.
In the conventional surface mountable coil described above, only small portions 6-1a, 6-2a of the base electrodes 6-1, 6-2 on the central parts of the end surfaces 2b, 3b of the core 4 are exposed through the encapsulating member 7. Therefore, the contact areas between the base internal electrodes 6-1, 6-2 and the terminal electrodes 8-1, 8-2 are limited to be the small portions of the base electrodes 6-1, 6-2 exposed through the encapsulating member 7, resulting in a structurally insufficient adhesion strength between the base and the terminal electrodes.
As a result, in case where the surface mountable coil 10 is soldered on a printed circuit board and subjected to thermal variation, e.g., by a thermal cycle test (TCT test), the terminal electrodes 8-1, 8-2 may be delaminated from contact portions of the base electrodes 6-1, 6-2, i.e., the exposed base electrodes 6-1a, 6-2a, due to thermally induced tensile stresses on the terminal electrodes 8-1, 8-2.
The present inventors have conducted a series of experiments and found that the mechanical contact strength between the base electrodes 6-1, 6-2 and the terminal electrodes 8-1, 8-2 can be substantially increased when the terminal electrodes 8-1, 8-2 are in contact with at least on portions of the peripheral surfaces 2a, 3a as well as the base electrodes 6-1, 6-2 on the end surfaces 2b, 3b. 
One may be tempted to remove parts of the encapsulating member 7 off the peripheral surfaces 2a, 3a after molding in order to expose the base electrodes 6-1, 6-2 underneath, but the encapsulating member 7 circumferentially formed thereon is too rigid to be readily removed.
Another way to expose the base electrodes 6-1, 6-2 on the peripheral surfaces 2a, 3a may be to remove a gap clearance between the inner surface of the mold and the base electrodes 6-1, 6-2 disposed on the peripheral surfaces 2a, 3a to prevent the synthetic resin from being injected through the gap during the molding process to reach the end surfaces 2b, 3b of the raised portion 2, 3. Since the gap serves as an escape path of the injected resin during the molding process, the core 4 and/or the wire 5 can be subjected to a high pressure induced by the absence of the escape path. The escape path is necessary for the synthetic resin to uniformly flow into and fill in the mold cavity, and consequently, burrs (surplus encapsulating member 7 on the peripheral surfaces 2a, 3a) would be unavoidably formed.
It is, therefore, an object of the present invention to provide a surface mountable coil having a reliable electrode structure, and method for the manufacture thereof.
In accordance with a preferred embodiment of the present invention, there is provided a surface mountable coil comprising:
a core including a body portion and two raised portions disposed at two opposite ends of the body portion, each of the raised portions having an end surface and a peripheral surface;
a winding wire wound around the body portion;
a pair of base electrodes, each of the base electrodes being disposed on the peripheral surface and the end surface of the raised portions, and two ends of the winding wire being connected to the base electrodes respectively;
an encapsulating member extending from a portion of one base electrode to a portion of the other base electrode to thereby cover the region therebetween while exposing a part of the base electrode on each peripheral surface and substantially the entire base electrode on each end surface; and
a pair of terminal electrodes respectively covering the exposed internal electrodes and the end portions of the encapsulating member on the peripheral surfaces of the raised portions,
wherein the end portions of the encapsulating member on the peripheral surfaces have peak portions extending toward the end surfaces of the raised portions and valley portions retracting away from the end surfaces.